The present invention relates to dual tone multi-frequency (hereinafter, "DTMF") detectors, and more particularly, to a DTMF detector for detecting a DTMF signal of 15 channels by using a digital signal processing (hereinafter, "DSP") chip.
A DTMF signal is a signal that includes two elemental signal tones each having a different frequency. A signal generated from a push-button telephone terminal is a typical example of a DTMF signal. In the United States, DTMF signalling usually employs one tone in a high frequency group (i.e., 1209, 1336, 1477 and 1633 Hz), and one tone from a lower frequency group (i.e., 697, 770, 852 and 941 Hz).
Several techniques for detecting DTMF signals exist in the prior art. One reference, U.S. Pat. No. 5,333,191 entitled Detection Of Multi-Frequency Tone Signals issued to McCarthy, discloses a method of operating a digital signal processor to detect DTMF tones in a digital voice telephone system in which digitally encoded signals appearing on a telephone channel are decimated to compress the spectrum to be monitored for the appearance of call signalling tones.
Another reference, U.S. Pat. No. 5,172,406 entitled DTMF Signal Detection Apparatus issued to Locke, discloses an apparatus for detecting and identifying DTMF signals in connection with voice store and forward (VSF) equipment. Locke '406 seeks to avoid the corruption of DTMF signals that are generated by a user while the voice store and forward VSF) equipment is transmitting signals (i.e., voice prompts or stored messages) to the user. While the foregoing prior art provides benefits in its own right, we believe that an improved technique for detecting DTMF signals can be contemplated.